Catheter-focused magnetic field induced renal nerve ablation

ABSTRACT

A flexible catheter includes a magnetically permeable element provided at its distal end. The magnetically permeable element is configured for placement within the renal artery. External coils, positionable on anterior and posterior portions of a patient in proximity to the renal artery, are coupled to a generator which energizes the external coils to create a high-frequency oscillating magnetic field in body tissue between the external coils including the renal artery and perivascular renal nerve tissue. The magnetically permeable element serves to concentrate the magnetic field in a region near the renal artery. The concentrated magnetic field induces high frequency electric current sufficient to ablate the perivascular renal nerve tissue proximate the renal artery. A cooling arrangement can be provided at the catheter&#39;s distal end and configured to provide cooling to the renal artery during ablation of the perivascular renal nerve tissue.

RELATED PATENT DOCUMENTS

This application claims the benefit of Provisional Patent ApplicationSer. No. 61/415,119 filed Nov. 18, 2010, to which priority is claimedpursuant to 35 U.S.C. §119(e) and which is incorporated herein byreference.

SUMMARY

Embodiments of the disclosure are directed to apparatuses and methodsfor ablating target tissue of the body using a concentrated magneticfield to induce high frequency electric current sufficient to ablate thetarget tissue. According to various embodiments, an apparatus includes aflexible elongated member having a proximal end, a distal end, and alength sufficient to access a target vessel of the body relative to apercutaneous access location. A magnetically permeable element isprovided at a distal end of the elongated member. The magneticallypermeable element has poor electrical conductivity and is configured forplacement within the target vessel and adjacent a wall of the targetvessel. One or more external coils are positionable on one or both ofanterior and posterior portions of a patient in proximity to the targetvessel. A generator is coupled to the external coils and configured toenergize the external coils to create a high-frequency oscillatingmagnetic field in body tissue between the external coils including thetarget vessel and target tissue proximate the target vessel. Themagnetically permeable element serves to concentrate the magnetic fieldin a region near the target vessel. The concentrated magnetic fieldinduces high frequency electric current sufficient to ablate the targettissue proximate the vessel.

According to some embodiments, an apparatus includes a flexibleelongated member having a proximal end, a distal end, and a lengthsufficient to access a renal artery relative to a percutaneous accesslocation. A magnetically permeable element is provided at a distal endof the elongated member. The magnetically permeable element has poorelectrical conductivity and is configured for placement within the renalartery and adjacent a wall of the renal artery. One or more externalcoils are positionable on one or both of anterior and posterior portionsof a patient in proximity to the renal artery. A generator is coupled tothe external coils and configured to energize the external coils tocreate a high-frequency oscillating magnetic field in body tissuebetween the external coils including the renal artery and perivascularrenal nerve tissue proximate the renal artery. The magneticallypermeable element serves to concentrate the magnetic field in a regionnear the renal artery. The concentrated magnetic field induces highfrequency electric current sufficient to ablate the perivascular renalnerve tissue proximate the renal artery. A cooling arrangement can beprovided at the distal end of the elongated member and configured toprovide cooling to the renal artery during ablation of the perivascularrenal nerve tissue.

In other embodiments, a method involves energizing one or more externalcoils positionable on one or both of anterior and posterior portions ofa patient in proximity to a renal artery to create a high-frequencyoscillating magnetic field in body tissue between the external coilsincluding renal artery tissue and perivascular renal nerve tissueadjacent the renal artery. The method also involves concentrating themagnetic field in a region near the renal artery, and ablating theperivascular renal nerve tissue using high frequency electric currentinduced in the perivascular renal nerve tissue by the concentratedmagnetic field. The method may further involve providing cooling to therenal artery during ablation of the perivascular renal nerve tissue.

These and other features can be understood in view of the followingdetailed discussion and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a right kidney and renal vasculatureincluding a renal artery branching laterally from the abdominal aorta;

FIGS. 2A and 2B illustrate sympathetic innervation of the renal artery;

FIG. 3A illustrates various tissue layers of the wall of the renalartery;

FIGS. 3B and 3C illustrate a portion of a renal nerve;

FIG. 4 illustrates a medical system for ablating target tissue of thebody, such as perivascular renal nerve tissue, using a concentratedmagnetic field to induce high frequency electric current sufficient toablate the target tissue in accordance with various embodiments;

FIG. 5 is a schematic representation of an externally generated magneticfield concentrated at a target region adjacent a renal artery by use ofa magnetic focusing catheter in accordance with various embodiments; and

FIG. 6 illustrates an apparatus for producing ablative heating ininnervated renal arterial and perivascular tissue provided from currentsinduced by an external magnetic field in accordance with variousembodiments.

DETAILED DESCRIPTION

Ablation of perivascular renal nerves has been used as a treatment forhypertension. Radiofrequency (RF) electrodes placed in the renal arterycan be used to ablate the nerves, but with risk of artery wall injury.To control injury to the artery wall, one approach is to ablate atdiscrete locations along and around the artery, repositioning theelectrode between locations. Devices with multiple RF electrodes havebeen proposed to avoid the need for multiple repositioning and ablationcycles, but these devices and their use are more complicated.

Even with ablation of discrete locations, renal artery injury in theselocations can occur due to local high temperatures resulting from highcurrent density near the electrodes. Many RF ablation approaches producea small focal area of heating, with the greatest heating at the arterywall. Away from the electrode, the current density and ohmic heatingfall off very rapidly. In order to effectively ablate the target tissue,a larger zone of heating, or multiple separate ablations at differentlocations, is needed.

Embodiments of the disclosure are directed to apparatuses and methodsfor effectively ablating target tissue of the body with reduced injuryto non-targeted tissue. Embodiments of the disclosure are directed toapparatuses and methods for ablation of perivascular renal nerves fortreatment of hypertension. Embodiments are directed to apparatuses andmethods for generating a high-frequency oscillating magnetic field inbody tissue that includes target tissue to be ablated, concentrating themagnetic field in a region near a magnetically permeable elementpositioned proximate the target tissue, and ablating the target tissueusing high frequency electric current induced in the target tissueproximate the magnetically permeable element by the concentratedmagnetic field. In some embodiments, the magnetically permeable elementis configured for placement within a vessel, such as the renal artery,and the target tissue is located proximate the vessel, such asperivascular renal nerve tissue adjacent the renal artery.

Various embodiments of the disclosure are directed to apparatuses andmethods for renal denervation for treating hypertension. Hypertension isa chronic medical condition in which the blood pressure is elevated.Persistent hypertension is a significant risk factor associated with avariety of adverse medical conditions, including heart attacks, heartfailure, arterial aneurysms, and strokes. Persistent hypertension is aleading cause of chronic renal failure. Hyperactivity of the sympatheticnervous system serving the kidneys is associated with hypertension andits progression. Deactivation of nerves in the kidneys via renaldenervation can reduce blood pressure, and may be a viable treatmentoption for many patients with hypertension who do not respond toconventional drugs.

The kidneys are instrumental in a number of body processes, includingblood filtration, regulation of fluid balance, blood pressure control,electrolyte balance, and hormone production. One primary function of thekidneys is to remove toxins, mineral salts, and water from the blood toform urine. The kidneys receive about 20-25% of cardiac output throughthe renal arteries that branch left and right from the abdominal aorta,entering each kidney at the concave surface of the kidneys, the renalhilum.

Blood flows into the kidneys through the renal artery and the afferentarterioles, entering the filtration portion of the kidney, the renalcorpuscle. The renal corpuscle is composed of the glomerulus, a thicketof capillaries, surrounded by a fluid-filled, cup-like sac calledBowman's capsule. Solutes in the blood are filtered through the verythin capillary walls of the glomerulus due to the pressure gradient thatexists between the blood in the capillaries and the fluid in theBowman's capsule. The pressure gradient is controlled by the contractionor dilation of the arterioles. After filtration occurs, the filteredblood moves through the efferent arterioles and the peritubularcapillaries, converging in the interlobular veins, and finally exitingthe kidney through the renal vein.

Particles and fluid filtered from the blood move from the Bowman'scapsule through a number of tubules to a collecting duct. Urine isformed in the collecting duct and then exits through the ureter andbladder. The tubules are surrounded by the peritubular capillaries(containing the filtered blood). As the filtrate moves through thetubules and toward the collecting duct, nutrients, water, andelectrolytes, such as sodium and chloride, are reabsorbed into theblood.

The kidneys are innervated by the renal plexus which emanates primarilyfrom the aorticorenal ganglion. Renal ganglia are formed by the nervesof the renal plexus as the nerves follow along the course of the renalartery and into the kidney. The renal nerves are part of the autonomicnervous system which includes sympathetic and parasympatheticcomponents. The sympathetic nervous system is known to be the systemthat provides the bodies “fight or flight” response, whereas theparasympathetic nervous system provides the “rest and digest” response.Stimulation of sympathetic nerve activity triggers the sympatheticresponse which causes the kidneys to increase production of hormonesthat increase vasoconstriction and fluid retention. This process isreferred to as the renin-angiotensin-aldosterone-system (RAAS) responseto increased renal sympathetic nerve activity.

In response to a reduction in blood volume, the kidneys secrete renin,which stimulates the production of angiotensin. Angiotensin causes bloodvessels to constrict, resulting in increased blood pressure, and alsostimulates the secretion of the hormone aldosterone from the adrenalcortex. Aldosterone causes the tubules of the kidneys to increase thereabsorption of sodium and water, which increases the volume of fluid inthe body and blood pressure.

Congestive heart failure (CHF) is a condition that has been linked tokidney function. CHF occurs when the heart is unable to pump bloodeffectively throughout the body. When blood flow drops, renal functiondegrades because of insufficient perfusion of the blood within the renalcorpuscles. The decreased blood flow to the kidneys triggers an increasein sympathetic nervous system activity (i.e., the RAAS becomes tooactive) that causes the kidneys to secrete hormones that increase fluidretention and vasorestriction. Fluid retention and vasorestriction inturn increases the peripheral resistance of the circulatory system,placing an even greater load on the heart, which diminishes blood flowfurther. If the deterioration in cardiac and renal functioningcontinues, eventually the body becomes overwhelmed, and an episode ofheart failure decompensation occurs, often leading to hospitalization ofthe patient.

FIG. 1 is an illustration of a right kidney 10 and renal vasculatureincluding a renal artery 12 branching laterally from the abdominal aorta20. In FIG. 1, only the right kidney 10 is shown for purposes ofsimplicity of explanation, but reference will be made herein to bothright and left kidneys and associated renal vasculature and nervoussystem structures, all of which are contemplated within the context ofembodiments of the disclosure. The renal artery 12 is purposefully shownto be disproportionately larger than the right kidney 10 and abdominalaorta 20 in order to facilitate discussion of various features andembodiments of the present disclosure.

The right and left kidneys are supplied with blood from the right andleft renal arteries that branch from respective right and left lateralsurfaces of the abdominal aorta 20. Each of the right and left renalarteries is directed across the crus of the diaphragm, so as to formnearly a right angle with the abdominal aorta 20. The right and leftrenal arteries extend generally from the abdominal aorta 20 torespective renal sinuses proximate the hilum 17 of the kidneys, andbranch into segmental arteries and then interlobular arteries within thekidney 10. The interlobular arteries radiate outward, penetrating therenal capsule and extending through the renal columns between the renalpyramids. Typically, the kidneys receive about 20% of total cardiacoutput which, for normal persons, represents about 1200 mL of blood flowthrough the kidneys per minute.

The primary function of the kidneys is to maintain water and electrolytebalance for the body by controlling the production and concentration ofurine. In producing urine, the kidneys excrete wastes such as urea andammonium. The kidneys also control reabsorption of glucose and aminoacids, and are important in the production of hormones including vitaminD, renin and erythropoietin.

An important secondary function of the kidneys is to control metabolichomeostasis of the body. Controlling hemostatic functions includeregulating electrolytes, acid-base balance, and blood pressure. Forexample, the kidneys are responsible for regulating blood volume andpressure by adjusting volume of water lost in the urine and releasingerythropoietin and renin, for example. The kidneys also regulate plasmaion concentrations (e.g., sodium, potassium, chloride ions, and calciumion levels) by controlling the quantities lost in the urine and thesynthesis of calcitrol. Other hemostatic functions controlled by thekidneys include stabilizing blood pH by controlling loss of hydrogen andbicarbonate ions in the urine, conserving valuable nutrients bypreventing their excretion, and assisting the liver with detoxification.

Also shown in FIG. 1 is the right suprarenal gland 11, commonly referredto as the right adrenal gland. The suprarenal gland 11 is a star-shapedendocrine gland that rests on top of the kidney 10. The primary functionof the suprarenal glands (left and right) is to regulate the stressresponse of the body through the synthesis of corticosteroids andcatecholamines, including cortisol and adrenaline (epinephrine),respectively. Encompassing the kidneys 10, suprarenal glands 11, renalvessels 12, and adjacent perirenal fat is the renal fascia, e.g.,Gerota's fascia, (not shown), which is a fascial pouch derived fromextraperitoneal connective tissue.

The autonomic nervous system of the body controls involuntary actions ofthe smooth muscles in blood vessels, the digestive system, heart, andglands. The autonomic nervous system is divided into the sympatheticnervous system and the parasympathetic nervous system. In general terms,the parasympathetic nervous system prepares the body for rest bylowering heart rate, lowering blood pressure, and stimulating digestion.The sympathetic nervous system effectuates the body's fight-or-flightresponse by increasing heart rate, increasing blood pressure, andincreasing metabolism.

In the autonomic nervous system, fibers originating from the centralnervous system and extending to the various ganglia are referred to aspreganglionic fibers, while those extending from the ganglia to theeffector organ are referred to as postganglionic fibers. Activation ofthe sympathetic nervous system is effected through the release ofadrenaline (epinephrine) and to a lesser extent norepinephrine from thesuprarenal glands 11. This release of adrenaline is triggered by theneurotransmitter acetylcholine released from preganglionic sympatheticnerves.

The kidneys and ureters (not shown) are innervated by the renal nerves14. FIGS. 1 and 2A-2B illustrate sympathetic innervation of the renalvasculature, primarily innervation of the renal artery 12. The primaryfunctions of sympathetic innervation of the renal vasculature includeregulation of renal blood flow and pressure, stimulation of reninrelease, and direct stimulation of water and sodium ion reabsorption.

Most of the nerves innervating the renal vasculature are sympatheticpostganglionic fibers arising from the superior mesenteric ganglion 26.The renal nerves 14 extend generally axially along the renal arteries12, enter the kidneys 10 at the hilum 17, follow the branches of therenal arteries 12 within the kidney 10, and extend to individualnephrons. Other renal ganglia, such as the renal ganglia 24, superiormesenteric ganglion 26, the left and right aorticorenal ganglia 22, andceliac ganglia 28 also innervate the renal vasculature. The celiacganglion 28 is joined by the greater thoracic splanchnic nerve (greaterTSN). The aorticorenal ganglia 26 is joined by the lesser thoracicsplanchnic nerve (lesser TSN) and innervates the greater part of therenal plexus.

Sympathetic signals to the kidney 10 are communicated via innervatedrenal vasculature that originates primarily at spinal segments T10-T12and L1. Parasympathetic signals originate primarily at spinal segmentsS2-S4 and from the medulla oblongata of the lower brain. Sympatheticnerve traffic travels through the sympathetic trunk ganglia, where somemay synapse, while others synapse at the aorticorenal ganglion 22 (viathe lesser thoracic splanchnic nerve, i.e., lesser TSN) and the renalganglion 24 (via the least thoracic splanchnic nerve, i.e., least TSN).The postsynaptic sympathetic signals then travel along nerves 14 of therenal artery 12 to the kidney 10. Presynaptic parasympathetic signalstravel to sites near the kidney 10 before they synapse on or near thekidney 10.

With particular reference to FIG. 2A, the renal artery 12, as with mostarteries and arterioles, is lined with smooth muscle 34 that controlsthe diameter of the renal artery lumen 13. Smooth muscle, in general, isan involuntary non-striated muscle found within the media layer of largeand small arteries and veins, as well as various organs. The glomeruliof the kidneys, for example, contain a smooth muscle-like cell calledthe mesangial cell. Smooth muscle is fundamentally different fromskeletal muscle and cardiac muscle in terms of structure, function,excitation-contraction coupling, and mechanism of contraction.

Smooth muscle cells can be stimulated to contract or relax by theautonomic nervous system, but can also react on stimuli from neighboringcells and in response to hormones and blood borne electrolytes andagents (e.g., vasodilators or vasoconstrictors). Specialized smoothmuscle cells within the afferent arterioles of the juxtaglomerularapparatus of kidney 10, for example, produces renin which activates theangiotension II system.

The renal nerves 14 innervate the smooth muscle 34 of the renal arterywall 15 and extend lengthwise in a generally axial or longitudinalmanner along the renal artery wall 15. The smooth muscle 34 surroundsthe renal artery circumferentially, and extends lengthwise in adirection generally transverse to the longitudinal orientation of therenal nerves 14, as is depicted in FIG. 2B.

The smooth muscle 34 of the renal artery 12 is under involuntary controlof the autonomic nervous system. An increase in sympathetic activity,for example, tends to contract the smooth muscle 34, which reduces thediameter of the renal artery lumen 13 and decreases blood perfusion. Adecrease in sympathetic activity tends to cause the smooth muscle 34 torelax, resulting in vessel dilation and an increase in the renal arterylumen diameter and blood perfusion. Conversely, increasedparasympathetic activity tends to relax the smooth muscle 34, whiledecreased parasympathetic activity tends to cause smooth musclecontraction.

FIG. 3A shows a segment of a longitudinal cross-section through a renalartery, and illustrates various tissue layers of the wall 15 of therenal artery 12. The innermost layer of the renal artery 12 is theendothelium 30, which is the innermost layer of the intima 32 and issupported by an internal elastic membrane. The endothelium 30 is asingle layer of cells that contacts the blood flowing though the vessellumen 13. Endothelium cells are typically polygonal, oval, or fusiform,and have very distinct round or oval nuclei. Cells of the endothelium 30are involved in several vascular functions, including control of bloodpressure by way of vasoconstriction and vasodilation, blood clotting,and acting as a barrier layer between contents within the lumen 13 andsurrounding tissue, such as the membrane of the intima 32 separating theintima 32 from the media 34, and the adventitia 36. The membrane ormaceration of the intima 32 is a fine, transparent, colorless structurewhich is highly elastic, and commonly has a longitudinal corrugatedpattern.

Adjacent the intima 32 is the media 33, which is the middle layer of therenal artery 12. The media is made up of smooth muscle 34 and elastictissue. The media 33 can be readily identified by its color and by thetransverse arrangement of its fibers. More particularly, the media 33consists principally of bundles of smooth muscle fibers 34 arranged in athin plate-like manner or lamellae and disposed circularly around thearterial wall 15. The outermost layer of the renal artery wall 15 is theadventitia 36, which is made up of connective tissue. The adventitia 36includes fibroblast cells 38 that play an important role in woundhealing.

A perivascular region 37 is shown adjacent and peripheral to theadventitia 36 of the renal artery wall 15. A renal nerve 14 is shownproximate the adventitia 36 and passing through a portion of theperivascular region 37. The renal nerve 14 is shown extendingsubstantially longitudinally along the outer wall 15 of the renal artery12. The main trunk of the renal nerves 14 generally lies in or on theadventitia 36 of the renal artery 12, often passing through theperivascular region 37, with certain branches coursing into the media 33to enervate the renal artery smooth muscle 34.

Embodiments of the disclosure may be implemented to provide varyingdegrees of denervation therapy to innervated renal vasculature. Forexample, embodiments of the disclosure may provide for control of theextent and relative permanency of renal nerve impulse transmissioninterruption achieved by denervation therapy delivered using a treatmentapparatus of the disclosure. The extent and relative permanency of renalnerve injury may be tailored to achieve a desired reduction insympathetic nerve activity (including a partial or complete block) andto achieve a desired degree of permanency (including temporary orirreversible injury).

Returning to FIGS. 3B and 3C, the portion of the renal nerve 14 shown inFIGS. 3B and 3C includes bundles 14 a of nerve fibers 14 b eachcomprising axons or dendrites that originate or terminate on cell bodiesor neurons located in ganglia or on the spinal cord, or in the brain.Supporting tissue structures 14 c of the nerve 14 include theendoneurium (surrounding nerve axon fibers), perineurium (surroundsfiber groups to form a fascicle), and epineurium (binds fascicles intonerves), which serve to separate and support nerve fibers 14 b andbundles 14 a. In particular, the endoneurium, also referred to as theendoneurium tube or tubule, is a layer of delicate connective tissuethat encloses the myelin sheath of a nerve fiber 14 b within afasciculus.

Major components of a neuron include the soma, which is the central partof the neuron that includes the nucleus, cellular extensions calleddendrites, and axons, which are cable-like projections that carry nervesignals. The axon terminal contains synapses, which are specializedstructures where neurotransmitter chemicals are released in order tocommunicate with target tissues. The axons of many neurons of theperipheral nervous system are sheathed in myelin, which is formed by atype of glial cell known as Schwann cells. The myelinating Schwann cellsare wrapped around the axon, leaving the axolemma relatively uncoveredat regularly spaced nodes, called nodes of Ranvier. Myelination of axonsenables an especially rapid mode of electrical impulse propagationcalled saltation.

In some embodiments, a treatment apparatus of the disclosure may beimplemented to deliver denervation therapy that causes transient andreversible injury to renal nerve fibers 14 b. In other embodiments, atreatment apparatus of the disclosure may be implemented to deliverdenervation therapy that causes more severe injury to renal nerve fibers14 b, which may be reversible if the therapy is terminated in a timelymanner. In preferred embodiments, a treatment apparatus of thedisclosure may be implemented to deliver denervation therapy that causessevere and irreversible injury to renal nerve fibers 14 b, resulting inpermanent cessation of renal sympathetic nerve activity. For example, atreatment apparatus may be implemented to deliver a denervation therapythat disrupts nerve fiber morphology to a degree sufficient tophysically separate the endoneurium tube of the nerve fiber 14 b, whichcan prevent regeneration and re-innervation processes.

By way of example, and in accordance with Seddon's classification as isknown in the art, a treatment apparatus of the disclosure may beimplemented to deliver a denervation therapy that interrupts conductionof nerve impulses along the renal nerve fibers 14 b by imparting damageto the renal nerve fibers 14 b consistent with neruapraxia. Neurapraxiadescribes nerve damage in which there is no disruption of the nervefiber 14 b or its sheath. In this case, there is an interruption inconduction of the nerve impulse down the nerve fiber, with recoverytaking place within hours to months without true regeneration, asWallerian degeneration does not occur. Wallerian degeneration refers toa process in which the part of the axon separated from the neuron's cellnucleus degenerates. This process is also known as anterogradedegeneration. Neurapraxia is the mildest form of nerve injury that maybe imparted to renal nerve fibers 14 b by use of a treatment apparatusaccording to embodiments of the disclosure.

A treatment apparatus may be implemented to interrupt conduction ofnerve impulses along the renal nerve fibers 14 b by imparting damage tothe renal nerve fibers consistent with axonotmesis. Axonotmesis involvesloss of the relative continuity of the axon of a nerve fiber and itscovering of myelin, but preservation of the connective tissue frameworkof the nerve fiber. In this case, the encapsulating support tissue 14 cof the nerve fiber 14 b is preserved. Because axonal continuity is lost,Wallerian degeneration occurs. Recovery from axonotmesis occurs onlythrough regeneration of the axons, a process requiring time on the orderof several weeks or months. Electrically, the nerve fiber 14 b showsrapid and complete degeneration. Regeneration and re-innervation mayoccur as long as the endoneural tubes are intact.

A treatment apparatus may be implemented to interrupt conduction ofnerve impulses along the renal nerve fibers 14 b by imparting damage tothe renal nerve fibers 14 b consistent with neurotmesis. Neurotmesis,according to Seddon's classification, is the most serious nerve injuryin the scheme. In this type of injury, both the nerve fiber 14 b and thenerve sheath are disrupted. While partial recovery may occur, completerecovery is not possible. Neurotmesis involves loss of continuity of theaxon and the encapsulating connective tissue 14 c, resulting in acomplete loss of autonomic function, in the case of renal nerve fibers14 b. If the nerve fiber 14 b has been completely divided, axonalregeneration causes a neuroma to form in the proximal stump.

A more stratified classification of neurotmesis nerve damage may befound by reference to the Sunderland System as is known in the art. TheSunderland System defines five degrees of nerve damage, the first two ofwhich correspond closely with neurapraxia and axonotmesis of Seddon'sclassification. The latter three Sunderland System classificationsdescribe different levels of neurotmesis nerve damage.

The first and second degrees of nerve injury in the Sunderland systemare analogous to Seddon's neurapraxia and axonotmesis, respectively.Third degree nerve injury, according to the Sunderland System, involvesdisruption of the endoneurium, with the epineurium and perineuriumremaining intact. Recovery may range from poor to complete depending onthe degree of intrafascicular fibrosis. A fourth degree nerve injuryinvolves interruption of all neural and supporting elements, with theepineurium remaining intact. The nerve is usually enlarged. Fifth degreenerve injury involves complete transection of the nerve fiber 14 b withloss of continuity.

In accordance with various embodiments, and with reference to FIG. 4,there is shown an apparatus 100 for ablating target tissue of the body.The apparatus 100 shown in FIG. 4 includes an external apparatus 101 andan apparatus 102 configured for placement within the body. The externaland internal apparatuses 101 and 102 cooperate to focus an externallygenerated magnetic field at a target region within the body andintensify the magnetic field near the target region resulting inincreased induced electric current and ohmic heating in target tissuewithin or near the target region. The ohmic heating is preferablycontrolled so that the target tissue is thermally ablated, butnon-targeted tissue is only negligibly affected.

As discussed previously above, conventional RF ablation approachesproduce a small focal area of heating, with the greatest heating at theartery. The current density and ohmic heating fall off by the 4^(th)power of the distance from the electrode center. Embodiments of thedisclosure are directed to approaches that generate a more uniformheating in the region adjacent to the artery, allowing more completeablation of target tissue with less artery wall injury.

According to the embodiment shown in FIG. 4, the internal apparatus 102includes a flexible elongated member 104 having a proximal end, a distalend, and a length sufficient to access a target vessel of the body, suchas a renal artery 12, relative to a percutaneous access location. Amagnetically permeable element 120 is provided at a distal end of theelongated member 104. The magnetically permeable element 120 preferablyhas low or poor electrical conductivity and is configured for placementwithin the renal artery 12 and adjacent a wall of the artery 12. Theinternal apparatus 102 shown in FIG. 4 is preferably delivered to therenal artery 12 via the inferior aorta 20 using one or both of a guidingcatheter and a delivery sheath. Alternatively, the internal apparatus102 can be implemented as a guiding catheter, and may incorporate alumen through which a guidewire can pass for advancing the apparatus 102through vasculature via an over-the-wire delivery technique, forexample. Marker bands or features can be placed on one or multiple partsof the elongated member 104, the magnetically permeable element 120,and/or a support arrangement 122 that contains or supports themagnetically permeable element 120. The marker bands or features aid theclinician in determining the location and position of specific portionsof the elongated member 104, such as the magnetically permeable element120.

The external apparatus 101 includes one or more external coils 105A and105B that can be positioned on one or both of anterior and posteriorportions of a patient in proximity to the renal artery 12. The externalapparatus 101 further includes a generator (shown in FIG. 6) coupled tothe external coils 105A and 105B. The generator is configured toenergize the external coils 105A and 105B to create a high-frequencyoscillating magnetic field in body tissue between the external coils105A and 105B including the renal artery 12 and perivascular renal nervetissue proximate the renal artery 12. The magnetically permeable element120 serves to concentrate the magnetic field in a region near the renalartery 12. The concentrated magnetic field induces high frequencyelectric current sufficient to ablate the perivascular renal nervetissue proximate the renal artery 12.

The internal apparatus 102 may include a support arrangement 122 at thedistal end of the elongated member 104. The support member 122preferably provides support for, or containment of, magnetic material ofthe magnetically permeable element 120. The magnetically permeableelement 120 may, for example, incorporate a vessel 122 filled with fluidcomprising magnetic material. The magnetically permeable element 120 mayincorporate a balloon 122 configured to receive a fluid comprisingmagnetic material from a lumen arrangement of the elongated member 104.The magnetically permeable element 120 can incorporate a balloon 122formed from a composite polymeric material comprising magnetic materialor a polymeric balloon 122 comprising a coating of nonconductive orpoorly conductive magnetic material. The magnetically permeable element120 may incorporate a balloon 122, and at least a circumferential regionof the balloon 122 is heated by the induced electric current, such thata complete circumferential region of the perivascular renal nerve tissueis ablated. The magnetically permeable element 120 may include a solidelement or a coated element 120 comprising nonconductive or poorlyconductive magnetic material. The magnetically permeable element 120 canincorporate a tube 122 containing magnetic material. Otherconfigurations of the magnetically permeable element 120 and supportarrangement 122 are contemplated.

Referring now to FIG. 5, there is illustrated a schematic representationof an externally generated magnetic field 110 which is beingconcentrated at a target region of the body adjacent a renal artery 12by use of a magnetic focusing catheter 102 according to variousembodiments of the disclosure. FIG. 5 shows a cross-section of the renalartery 12 within which a spherical vessel 122 containing a ferrofluid120 is positioned. In FIG. 5, the renal artery 12 is oriented coming outof the page. The electric field lines 135 around the spherical vessel122 containing the ferrofluid 120 are illustrated for simplicity. Thespherical vessel 122 that contains a ferrofluid 120 represents one ofmany possible configurations of a magnetically permeable element 120 andsupport arrangement 122, and is shown for non-limiting illustrativepurposes. For example, a cylindrical or annular ferrofluid volume may beprovided at a distal end or tip of a catheter shaft 104, with thecurrent loops displaced to the side since the ferrofluid isnonconductive. In FIG. 5, the magnetic field 110 is illustrated as beingconcentrated by a factor of about 2 for clarity of illustration.However, concentration of the magnetic field 115 proximate theferrofluid filled vessel 122 by ratios of magnetic permeability in therange of about 5 to 15 are typically achieved.

With reference to FIGS. 4 and 5, and in accordance with variousembodiments, a balloon catheter 102 is placed in a renal artery 12 andthe balloon 122 is filled with a ferrofluid 120. External coils 105A and105B are placed on the front and the back of the patient and energizedto create a high-frequency oscillating magnetic field 110 in the tissuebetween the coils 105A and 105B. The ferrofluid 120 concentrates themagnetic field 115 in the region near the catheter balloon 122preferably by approximately a factor of 10. The changing magnetic field115 induces a corresponding electric field 135 and results in highfrequency electric current in the tissues adjacent the balloon 122. Thecurrent causes local resistive heating in the tissue, proportional tothe square of the current, resulting in local heating near the balloon122 approximately 100 times that resulting in the other areas of tissuesubject to the magnetic field 110.

In this representative embodiment, tissue around the entirecircumference of the balloon 122 is heated by the induced current,avoiding the need for repeated catheter positioning and activation aswith typical RF devices. The magnetically induced heating is also moreuniform, since the current is affected primarily by the local magneticfield strength, which can fall off relatively slowly with separationfrom a magnetically permeable balloon 122. More uniform heating in theregion adjacent to the renal artery 12 is achieved, allowing morecomplete ablation of perivascular renal nerve tissue with less arterywall injury.

The following comparison between conventional RF catheter approaches andablation approaches consistent with embodiments of the disclosure isprovided below for illustrative purposes. Conventional RF catheterapproaches involve pressing a tip electrode against the arterial tissueand application of a given dose of RF power. Ablation current flows intothe arterial tissue from a portion of the tip electrode. The impedancebetween the tip electrode and ground pad consists of contact resistance,tissue resistance, and tissue capacitance. To estimate some ablationparameters, consider a simplified model of the RF ablation. The ablationtip can be modeled as a spherical electrode surrounded by tissue havinguniform electrical resistivity. It is assumed for illustrative purposesthat about ⅓ of the current passes into the arterial wall, while theremaining current is lost to the blood. The voltage at a point inarterial tissue a distance “r” from the spherical tip electrode is givenby:V=V ₀(r ₀ /r)  (1)where

-   -   V=voltage at distance r in volts    -   V₀=voltage applied to tip electrode, volts    -   r₀=tip electrode radius, meters    -   r=distance from center of tip electrode to point in tissue, in        meters        The electric field developed in the tissue is given by:        E=dV/dr=−V ₀(r ₀ /r ²)  (2)        where the electric field is directed radial outward from the tip        electrode. The current density is then given by Ohms law:        J=E/ρ=V ₀(r ₀ /ρr ²)  (3)        where    -   J=current density in Amps/m²    -   ρ=resistivity in Ohm-m        The total power dissipated in the arterial wall is given by:        P _(w) =∫d ³ rJE  (4)        where    -   P_(w)=power dissipated in artery wall in Watts    -   ∫d³r=integral over the tissue volume

To estimate the power dissipated in the arterial wall, an estimate ismade that about ⅓ of the tip electrode area is against the artery wall,so the result is about one third of the integration over all of space.Since the integrand falls off with the inverse fourth power of distance,most of the heat is dissipated close to the electrode, and theintegration can be extended over all space with the result approximatelyequal to the heat dissipated in tissue near the tip electrode.Substituting Equation (2) and (3) into Equation (4) above, and carryingout the integration yields:P _(w)=(4πr ₀/6ρ)V ₀ ²  (5)where a factor of three reduction is included to account for thearterial wall contacting one third of the tip electrode area, and afactor of two reduction is included to account for RMS value of the sinewave, RF current.

The resistivity of muscle and blood are similar, about 2 Ohm-m. The tipelectrode for renal nerve ablation has a diameter of about 1.5 mm. Thenominal voltage amplitude during an RF ablation is about 40 volts(varies with tissue impedance for constant power ablations). Using thesevalues in Equation (5) above yields P_(w)=1.3 Watts. This means that 2.6Watts is dissipated in blood. The constant power setting in the RFablation is 8 Watts. Thus, about half of this power, or 4.1 Watts, isdissipated in the leads and contact resistance between the tip electrodeand tissue.

In accordance with embodiments of the disclosure, ablative heating ofinnervated renal arterial and perivascular tissue is provided fromcurrents induced by an external magnetic field 110. In accordance withthe representative geometry schematically illustrated in FIG. 5, avoltage is induced in tissue adjacent the magnetically permeableferrofluid given by:V=dΦ/dt=ωμB ₀ A  (6)where

-   -   V induced voltage in volts    -   Φ=magnetic flux in webers    -   ω=2πf, where f is the frequency of the sine wave applied        magnetic field in Hz    -   μ=magnetic permeability of the ferrofluid    -   B₀=amplitude of the applied field in Tesla    -   A=area of a surface near the balloon

To simplify the calculation, consider currents induced to flow aroundthe circumference of a circle of diameter “d” that lies in the planeperpendicular to the applied magnetic field 110 of FIG. 5, with theplane of the circle near the ferrofluid 120. The voltage induced aroundthis circle is given by Equation (7) below as follows:V=(π²/2)μfB ₀ d ²  (7)The resistance around the circular path of tissues lying within a circleof diameter “d” is estimated by a circular current path with lengthequal to the circumference at half of the circle radius, or πd/2, and anarea equal to the radius of the circle times the width, w, of theconductive tissue, orR=πρ/w  (8)The heat generated in the tissue due to the induced currents is thengiven by:P=V ²/2R=[(π²/2)μfB ₀ d ²]² w/2πρ  (9)

Using the estimate from the RF ablation analysis below Equation (5) thatthe Ohmic power needed to ablate the tissue is 1.3 Watts, and taking thediameter of the circle equal to the width of the tissue ablated and bothequal to 4 mm (as estimates of the tissue volume ablated in renal nerveRF ablation, taken from Haines, Biophysics of Radiofrequency LesionFormation, Ch. 1, FIGS. 1-2), then solving Equation (9) above for μ f B₀gives:μfB ₀=8×10⁵  (10)

The permeability generally decreases with increasing frequency. As anestimate of this product, it is assumed that μ=10 at f=500 kHz can beachieved. In this case, the amplitude of the applied magnetic fieldneeded for ablation by the induced currents is on the order of 0.15Tesla. This is a large, but achievable magnetic field using water cooledcoils 105A and 105B, for example. Coils 105A and 105B may be placed bothabove and below the patient over the approximate location of the renalartery 12, as shown in FIG. 4.

Tissue that is in the magnetic field but not adjacent the ferrofluid 120will have induced currents with μ=1 in Equation (9) above. In thisillustrative example, this will result in heating at a power level thatis 100 times less than the tissue adjacent the ferrofluid 120, andpresumably well below the level needed to significantly raise thetemperature of these tissues. Also, the mixture of conductive andnon-conductive body tissues, for example fat, may greatly reduce theconductivity of adjacent tissues, and may break up or prevent closedcurrent paths in other tissues.

The power dissipated in the tissue adjacent the ferrofluid filledballoon 122 can be estimated by Equation (9) above. The heating powerincreases with the square of frequency, so higher frequency is desired,provided that the permeability is adequate. Ferrofluids 120 that consistof particles having a single magnetic domain may have higherpermeability at high frequencies than multi-domain particle.

The magnetic particles in the ferrofluid 120 should be physically smallto avoid induction of electrical currents within the particles thatwould generate heat within the balloon 122. Equation (9) above showsthat the heating power is proportional to the fourth power of particlediameter, so conventional ferrofluids 120 having sub-micron sizedparticles should not heat, even though they are much less electricallyresistive than body tissues. The particles need to be coated with anelectrical insulating material, and suspended in an electricallyinsulating solvent.

A ferrofluid 120 having a large permeability compared to body tissues isdesired so that heating occurs around the ferrofluid balloon 122, butnot in other body tissues. Additionally, induced electric fields 135that are spatially uniform in non-magnetic body tissues can be created,but non-uniform near the ferrofluid 120. This may be achieved, forexample, using the rotating magnetic field of a bird cage winding (e.g.,MRI RF bird cage), which creates a spatially uniform rotating electricfield.

Near the ferrofluid balloon 122 the magnetic field 115 is distorted, andtime varying electric fields 135 appear that circulate in the tissuenear the balloon 122 at the rotation rate of the applied field. Heatingwill occur preferentially in tissues around the balloon 122 but not intissues away from the balloon 122. Various distortions 115 of theapplied field 110 and various patterns of induced electric currents inbody tissue are contemplated using both paramagnetic materials anddiamagnetic materials (e.g., bismuth) in geometries that optimizeheating. Solid magnetic materials may be coated onto a balloon ordelivered within a tube that can be coiled up against the artery wall.

In accordance with other embodiments, a balloon 122 can be inflated withferrofluid 120. A double balloon 122 can be used in some embodiments,with the annulus inflated with ferrofluid 120. The inner balloon can beinflated with a conventional fluid. In other embodiments, a thinnon-conductive coating of magnetic material can be used. The balloon 122or other structure (e.g., catheter shaft 104 or expandable elementsupported at the distal end of the catheter shaft 104) can be coveredwith the thin non-conductive coating of magnetic material.

In various embodiments, a balloon 122 can be constructed of magneticnanoparticles in a polymer matrix to produce a nonconductive magneticballoon. The balloon 122 may be cooled to prevent heating of thearterial wall adjacent the balloon 122. Cooling may be provided by bloodflowing through the center of the balloon (e.g., a perfusion balloon).Other cooling mechanisms include circulating cooling fluids andapplication of cryogenic fluids as are known in the art. The balloon 122may contain a temperature sensor or thermometer 127 (shown in FIG. 6),for example one or more thermocouples, to monitor the temperatureadjacent the balloon 122 and provide feedback to automatically orsemi-automatically control the strength of the external magnetic field110 during ablation.

According to some embodiments, and as an alternative to a ferrofluidballoon 122, a catheter 102 tipped with a ferrite or other magneticallypermeable material having low electrical conductivity may be used. Theferrite is preferably pressed against the wall of the renal artery 12,and the external magnetic field 110 is applied. A thermometer 127 can beplaced adjacent the artery wall in a hole through the ferrite tip totitrate the external field. A generally uniform magnetic field 110 canbe obtained from external coils 105A and 105B, or a more concentratedmagnetic field can be obtained with modified external components. Themagnetic material on the catheter 102 can be used to orient the externalcomponents to maximize the field strength at the target region.

Turning now to FIG. 6, there is illustrated an apparatus 100 forproducing ablative heating in innervated renal arterial and perivasculartissue provided from currents induced by an external magnetic field inaccordance with various embodiments. In FIG. 6, the apparatus 100includes a generator 125 electrically coupled to a pair of externalcoils 105A and 105B positioned on respective anterior and posteriorportions of a patient in proximity to the renal artery 12. The externalcoils 105A and 105B may incorporate fluid channels or be thermallycoupled to fluid channels for cooling the external coils 105A and 105B.The fluid channels of the external coils 105A and 105B are shown fluidlycoupled to a coolant source 160 via lines 162A and 162B, respectively.Each of the coolant lines 162A and 162B preferably includes a supplyline and a return line, with each of the supply lines fluidly coupled toa pump 162 of the coolant source 160.

FIG. 6 further shows a magnetic focusing catheter 102 which includes amagnetically permeable element 120. In some embodiments, themagnetically permeable element 120 is supported by, or contained within,a support arrangement 122 (e.g., balloon, vessel, tube). In embodimentsthat employ a balloon 122, an inflation lumen 106 is provided whichextends along the length of the shaft 104 and is fluidly coupled to apump 162 via a line 113. The support arrangement 122 is configured toposition the magnetically permeable element 120 against the wall of therenal artery 12. The support arrangement 122 or other portion of thedistal end of the shaft 104 preferably incorporates a perfusion ordiversion apparatus to allow blood to flow through the renal artery 12during the ablation procedure. In some embodiments, as previouslydiscussed, the perfusion or diversion apparatus can be used to providecooling to the renal artery wall during ablation.

In other embodiments, the support arrangement 122 or other structure atthe distal end of the shaft 104 can be configured to receive a coolantor cryogen from an external coolant source 160 via a line 112 and acoolant lumen 108 of the shaft 104. For example, the support arrangement122 can incorporate a cyroballoon or cryotube. A cryogen can bedelivered from the coolant source 160 to the cyroballoon or cryotube ofthe support arrangement 122. In some implementations, a cryoballoon orcryotube can be configured to receive a liquid biocompatible cryogen,such as cold sterile saline or cold Ringer's solution, which is expelledinto the blood flowing through the renal artery 12. In otherimplementations, the cryoballoon or cryotube can be constructed toprovide phase-change cryothermal cooling by incorporating one or moreorifices or narrowings to induce a phase change in a liquid cryogensupplied to the cryoballoon (e.g., Joule-Thomson cooling). Spent gasresulting from the phase change of the liquid cryogen is exhaustedthrough an outlet fluidly coupled to an exhaust lumen extends to theproximal end of the catheter shaft 104. One or more temperature sensors127 may be provided at the support arrangement 122 for measuringtemperature approximating that of the renal artery wall adjacent themagnetically permeable element 120.

The generator 125 includes an oscillator configured to energize theexternal coils 105A and 105B to create a high-frequency oscillatingmagnetic field in body tissue between the external coils 105A and 105Bincluding the renal artery 12 and perivascular renal nerve tissueproximate the renal artery 12. The generator 125 may include or becoupled to a controller 129 which controls the generator 125 in anautomatic mode, semi-automatic mode or manual mode. The controller 129is preferably coupled to one or more temperature sensors 127 whichgenerate sensor signals indicative of the temperature at the renalartery wall adjacent the magnetically permeable element 120. Thecontroller 129 can be programmed to automatically adjust the magneticfield produced between the external coils 105A and 105B based on thesensor signals received from the temperature sensors 127.

In general, when renal artery nerve tissue temperatures rise above about113° F. (50° C.), protein is permanently damaged (including those ofrenal nerve fibers). If heated over about 65° C., collagen denatures andtissue shrinks. If heated over about 65° C. and up to 100° C., cellwalls break and oil separates from water. Above about 100° C., tissuedesiccates. According to some embodiments, the generator 125 isconfigured to control the magnetic field produced between the externalcoils 105A and 105B in response to signals received from temperaturesensors 127 so that an intensified magnetic field created near themagnetically permeable element 120 causes heating of the perivascularrenal nerve tissue 14 proximate the magnetically permeable element 120to at least a temperature of 55° C.

Various embodiments disclosed herein are generally described in thecontext of ablation of perivascular renal nerves for control ofhypertension. It is understood, however, that embodiments of thedisclosure have applicability in other contexts, such as performingablation from within other vessels of the body, including otherarteries, veins, and vasculature (e.g., cardiac and urinary vasculatureand vessels), and other tissues of the body, including various organs.Embodiments of the disclosure can be used for ablation of perivascularrenal nerves, or for ablation of other tissues where a ferrofluid-filledcatheter or elongated member or trocar can be placed for focusing theexternal magnetic field (e.g., diseased tissue, tumors, and organs). Forexample, for cardiac arrhythmia ablation, a catheter can be introducedthrough the vasculature and positioned in the left atrium. In anotherexample, for treatment of BPH (benign prostatic hyperplasia), a cathetercan be introduced through the urethra and positioned in the prostate.

It is to be understood that even though numerous characteristics ofvarious embodiments have been set forth in the foregoing description,together with details of the structure and function of variousembodiments, this detailed description is illustrative only, and changesmay be made in detail, especially in matters of structure andarrangements of parts illustrated by the various embodiments to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. A system, comprising: an internal apparatusintended to be deployed within a patient, the internal apparatusincluding: a flexible elongated member comprising a proximal end, adistal end, and a length sufficient to access a target vessel of thebody relative to a percutaneous access location; and a magneticallypermeable element provided at a distal end of the elongated member, themagnetically permeable element having poor electrical conductivity andconfigured for placement within the target vessel and adjacent a wall ofthe target vessel; and an external apparatus intended to be deployedoutside of the patient, the external apparatus including: one or moreexternal coils positionable on one or both of anterior and posteriorportions of a patient in proximity to the target vessel; and a generatorcoupled to the external coils and configured to energize the externalcoils to create a high-frequency oscillating magnetic field in bodytissue between the external coils including the target vessel and targettissue proximate the target vessel; wherein the magnetically permeableelement serves to concentrate the magnetic field, created by theexternal apparatus, in a region near the target vessel, the concentratedmagnetic field inducing high frequency electric current sufficient toablate the target tissue proximate the vessel.
 2. The system of claim 1,wherein the magnetically permeable element comprises a vessel filledwith fluid comprising magnetic material.
 3. The system of claim 1,wherein the magnetically permeable element comprises a balloon, theballoon configured to receive a fluid comprising magnetic material. 4.The system of claim 1, wherein the magnetically permeable elementcomprises a balloon, the balloon formed from a composite materialcomprising magnetic material or comprising a coating of nonconductive orpoorly conductive magnetic material.
 5. The system of claim 1, wherein:the magnetically permeable element comprises a balloon; at least acircumferential region of the balloon is heated by the induced electriccurrent; and a complete circumferential region of the target tissue inproximity to the circumferential region of the balloon is ablated. 6.The system of claim 1, wherein the magnetically permeable elementcomprises a solid element or a coated element, the solid or coatedelement comprising nonconductive or poorly conductive magnetic material.7. The system of claim 1, wherein the magnetically permeable elementcomprises a tube containing magnetic material.
 8. The system of claim 1,wherein the magnetically permeable element comprises paramagneticmaterial and diamagnetic material arranged in geometries that enhanceheating of the target tissue.
 9. The system of claim 1, wherein themagnetically permeable element is configured to cooperate with anexternal bird cage coil that generates a rotating magnetic field, themagnetically permeable element configured to distort a magnetic fieldproximate the element so that time varying electric fields are producedthat circulate in the target tissue at the rotation rate of rotatingmagnetic field.
 10. The system of claim 1, further comprising atemperature sensor arranged to sense temperature proximate themagnetically permeable element.
 11. The system of claim 1, furthercomprising a cooling arrangement at the distal end of the elongatedmember and configured to provide cooling to the target vessel duringablation of the target tissue.
 12. The system of claim 1, wherein themagnetically permeable element is configured to create a circumferentiallesion in the target tissue.
 13. The system of claim 1, wherein theconcentrated magnetic field is approximately 5-15 times greater thanthat in other regions of the body tissue between the external coils. 14.The system of claim 1, wherein the induced current causes localresistive heating near the magnetically permeable element approximately≧100 times that in other areas of the body tissue subject to themagnetic field.
 15. The system of claim 1, further comprising a coolingarrangement configured to cool the one or more external coils.
 16. Asystem comprising: a flexible elongated member comprising a proximalend, a distal end, and a length sufficient to access a renal arteryrelative to a percutaneous access location; a magnetically permeableelement provided at a distal end of the elongated member, themagnetically permeable element having poor electrical conductivity andconfigured for placement within the renal artery and adjacent a wall ofthe renal artery, the magnetically permeable element deployable within apatient; one or more external coils positionable on one or both ofanterior and posterior portions of the patient, exterior to the patient;in proximity to the renal artery; and a generator coupled to theexternal coils and configured to energize the external coils to create ahigh-frequency oscillating magnetic field in body tissue between theexternal coils including the renal artery and perivascular renal nervetissue proximate the renal artery; the magnetically permeable elementserving to concentrate the magnetic field in a region near the renalartery, the concentrated magnetic field inducing high frequency electriccurrent sufficient to ablate the perivascular renal nerve tissueproximate the renal artery.
 17. The system of claim 16, furthercomprising a cooling arrangement at the distal end of the elongatedmember and configured to provide cooling to the renal artery duringablation of the perivascular renal nerve tissue.
 18. The system of claim16, further comprising a cooling arrangement configured to cool the oneor more external coils.
 19. The system of claim 16, wherein themagnetically permeable element is configured to create a circumferentiallesion in the perivascular renal nerve tissue.
 20. The system of claim16, wherein: the magnetically permeable element comprises a balloon; andthe balloon comprises a cooling arrangement configured to providecooling to the renal artery during ablation of the perivascular renalnerve tissue.
 21. A method, comprising: energizing one or more externalcoils positionable on one or both of anterior and posterior portions ofa patient in proximity to a renal artery to create a high-frequencyoscillating magnetic field in body tissue between the external coilsincluding renal artery tissue and perivascular renal nerve tissueadjacent the renal artery; concentrating the magnetic field in a regionnear the renal artery; and ablating the perivascular renal nerve tissueusing high frequency electric current induced in the perivascular renalnerve tissue by the concentrated magnetic field.
 22. The method of claim21, wherein: concentrating the magnetic field comprises concentratingthe magnetic field in a region near a magnetically permeable elementsituated in the renal artery; and ablating the perivascular renal nervetissue comprises ablating the perivascular renal nerve tissue using highfrequency electric current induced in the perivascular renal nervetissue proximate the magnetically permeable element by the concentratedmagnetic field.
 23. The method of claim 21, further comprising providingcooling to the renal artery during ablation of the perivascular renalnerve tissue.